In a heat exchanger of the above type, the first fluid flows from one chamber to another through apertures in the heat exchanger plates, so as to exchange heat with the second fluid flowing in the passages defined between the pairs of plates. Such a heat exchanger is currently used as an evaporator in a coolant fluid circuit for air conditioning the cabin of a motor vehicle, in which the refrigerant fluid is the above mentioned first fluid, the second fluid being atmospheric air.
In general terms, each plate of the heat exchanger is in the form of a shallow tray, and is formed with two holes which act as the inlet and outlet, respectively, for the first fluid. The chamber which is defined between the two plates of any one pair of plates includes an internal partition which gives the fluid flowing in the chamber a U-shaped flow path between the inlet aperture and the outlet aperture. This partition is generally formed by sealingly joining together two longitudinal projecting ribs each of which forms part of a respective one of the plates in the pair of plates concerned. The communicating apertures are usually formed in a projecting element or pocket which is arranged at one end of each plate. In addition, the plates are generally joined together at their other end by a base plate which provides spacing between the pairs of plates.
Where the heat exchanger is an evaporator, the refrigerant fluid enters the heat exchanger in the liquid state and leaves it in the vapour state, after having cooled a stream of air by evaporation. Since this stream of air contains a certain amount of humidity, condensation occurs in contact with the heat exchanger plates, giving rise to trickling of water. It is desirable to eliminate this water. In known heat exchangers of the stacked plate type, this water has a tendency to stagnate on the base plate of the evaporator, and this can give rise to corrosion.